There is a strong resurgence of methamphetamine and other stimulant use in the United States (US). HIV- positive stimulant-using men who have sex with men (HIV+ SUMSM) may have greater difficulties navigating the HIV care continuum as well as display substantially elevated viral load (VL), amplified HIV transmission risk, and faster clinical HIV progression. While behavioral interventions show promise for achieving durable reductions in unsuppressed VL over time among HIV+ SUMSM, those residing outside of urban centers experience difficulties accessing services for HIV and substance use. For these reasons we propose a two-arm RCT of a 6-month mHealth intervention for HIV+ SUMSM, called Supporting Treatment Adherence for Resilience and Thriving (START). We will test the efficacy of START to improve VL suppression at 6 months (primary outcome) and determine whether any gains are maintained at month 12 (secondary outcome). START integrates two theoretically-grounded, evidence-based interventions (APP+ and ARTEMIS) with the goal of optimizing the effectiveness of treatment as prevention (TasP). HIV+ SUMSM (n=350) will be recruited and randomized to START or control to assess the following aims: Aim 1a. Test the efficacy of START for achieving a higher proportion of SUMSM who are virally suppressed at 6 months (primary outcome) compared to a website with referrals to HIV treatment information and substance use treatment resources (control condition). Aim 1b. Test the efficacy of START for maintaining viral suppression gains at 12 months, decreasing stimulant use and sexual risk, and increasing theory-based psychological processes (e.g., behavioral skills, positive affect). Aim 2. To assess the cost and cost-effectiveness of START, relative to the control condition, in achieving and/or maintaining viral suppression, including net savings with respect to averted healthcare utilization. During Phase I, we will integrate APP+ and ARTEMIS into a single START intervention platform using an iterative integration process that includes feedback through online focus groups with SUMSM. Phase I will conclude with usability testing among 10 HIV+ SUMSM to ensure a smooth transition to Phase II where we will conduct an RCT to test the efficacy of START. We will use home-based dried blood spot (DBS) collection to assess lab-quantified VL. All men will receive their respective condition for 6 months, with quarterly assessments until month 12. DBS specimens to measure VL will be collected at baseline, 6 and 12 months. The cost analysis will be framed from the provider perspective to estimate ?real world? costs of providing START to give stakeholders a sense of feasibility for broader implementation, given existing resources and reimbursement mechanisms. The proposed project is highly significant since optimizing TasP with HIV+ SUMSM is among the highest NIH and National HIV/AIDS Strategy priorities. START is innovative because it is scalable to reach a broader population of HIV+ SUMSM and may be adapted to clinic- and community-based settings.